We propose to compare the changes in: 1) Skeletal muscle properties; and 2) Endurance exercise performance, in response to endurance training in young (20-30 years) versus elderly (60-65) subjects. Aging is associated with loss of physical endurance capacity and a decline in habitual physical activity. We will determine the degree of association between endurance capacity and oxidative enzyme activity before and after endurance training. The subjects will be trained for 12 weeks on cycle ergometers at a power output which elicits a heart rate at 65% of the range between resting and maximum heart rate (heart rate reserve, HRR). Training induced changes of endurance capacity will be assessed by measuring VO2max and the responses (rating of perceived exertion, minute ventilation, heart rate and blood lactate) to single load submaximal exercise. Muscle will be sampled from the vastus lateralis before and after training and oxidative enzyme activity (SDH, succinate dehydrogenase) and glycolytic (alpha-glycerophosphate dehydrogenase) enzyme activities within individual fibers will be determined using quantitative histochemical methods. Previous studies of the effect of aging on skeletal muscle have employed muscle homogenates. This approach may be inappropriate since age related changes in muscle fiber composition may influence muscle enzyme activities measured in a homogenate. The quantitative histochemical procedures will permit us to determine oxidative and glycolytic activity within defined fiber types (Type I, IIa and IIb). This pilot study will provide important information concerning the distribution, in slow and fast twitch fibers, of enzyme activity in young and elderly subjects. Pilot data is required to determine if oxidative enzyme activity is correlated with endurance (maximal oxygen uptake or responses to submaximal exercise) before and after training. Low intensity exercise is recommended for seniors; however, the minimum intensity which will provoke a training response is unknown. Pilot data from this study will allow determination of the sample size required to evaluate the effects on muscle enzyme activity and endurance capacity of different training intensities in the elderly. Defining the minimum effective training intensity is important for the elderly since high intensity exercise is associated with increased risk of cardiovascular or orthopaedic injury.